This invention relates to sidelobe cancelling and more particularly to an intermediate frequency sidelobe canceller for avoidance of low duty-cycle pulsed radar jamming.
Conventional IF sidelobe cancellers were developed to cancel barrage noise, or "brute-force" jamming. The fact that they require a few microseconds to "lock-on" before beginning to cancel barrage noise is of no consequence since the interference is intended to obliterate the radar received skin returns over a period of time. However, with the advent of deception jamming, isolated short pulses can be transmitted by a standoff jammer which penetrate to the radar receiver. This is done by making the pulse duration comparable with the loop lock-up time and spacing the pulses far enough apart to allow the error signal to decay to essentially zero between pulses. In a typical search radar utilizing a 5 microsecond pulse and using a conventional IF canceller, the integrating filter may have a bandwidth of 7000 Hz. This corresponds to a lock-on time of about 5 microseconds for a jamming pulse approximately 30 dB above receiver noise. The decay time of the error signal in the filter is approximately 100 microseconds. Therefore, if a standoff jammer emits 5 microsecond rf pulses spaced by more than 100 microseconds they will pass through the sidelobe canceller and appear on the radar scope as false targets. The "prelocked" or delayed channel method about to be described can provide a means whereby pulses of length in the order of the loop lock-up time may be cancelled irrespective of jammer pulse-to-pulse spacing.